In recent years, the power output of field-effect transistors (FETs) and bipolar transistors for microwave frequency applications has been increased, so that nowadays microwave power of 20 W and over can be obtained with a single transistor. However, since the working voltage of such a transistor is as low as about 10 V, a large DC bias current of 5 to 8A is needed.
On the other hand, for bias supply circuits, there has been a demand for DC current to be supplied via a microstrip line of high characteristic impedance in order to minimize the effect in the frequency range in which the microwave circuit is used. Maintaining such a high characteristic impedance, however, requires reducing the width of the conductor strip.
Accordingly, there is a demand to provide a microstrip line that permits a large DC current supply while maintaining a high characteristic impedance.
To meet such a demand, it has been practiced in the prior art to bond a fine gold wire or gold ribbon to a conductor strip of high characteristic impedance to achieve a reduction in DC resistance.
However, such a technique has only been effective in permitting a current of a magnitude two to three times greater than when the conductor strip is used alone, and has not been able to supply sufficient DC bias current for a high power transistor that requires a large current.
Furthermore, in a monolithic microwave circuit that is formed simultaneously with a large number of active and passive elements on a semiconductor substrate, the above technique of bonding a gold wire or gold ribbon to a conductor strip cannot be employed because of dimensional constraints.
In Japanese Unexamined Patent Publication No. 1-158801, there is disclosed a microstrip line comprising a conductor strip having a T-shaped cross section with the top width at least two times greater than the bottom width. The upper overhang provides a conductor cross section that allows a large DC current, and yet, its contribution to the characteristic impedance is small because of the presence of an air layer between the overhang and the dielectric substrate. Thus, with the T-shaped cross section, the resulting microstrip line has a high characteristic impedance and, at the same time, is capable of handling a sufficiently large DC current flow.
However, although the contribution of the overhang to the characteristic impedance is small, the contribution may increase to a degree that cannot be neglected if the area of the overhang is large and if the distance to the dielectric substrate is small. According to the method disclosed in the above patent publication for the fabrication of a microstrip line having a T-shaped cross section, which method combines a layer formation process with a photoetching process, the distance between the overhang and the dielectric substrate is inevitably reduced, and also, the area of the overhang increases as the top width is enlarged to permit a larger current flow. Thus, the prior art method has a problem that there is a limit to the allowable current value because the characteristic impedance decreases if the T-shaped cross section is so formed as to allow a larger current flow.